1. Field of the Invention
The present invention relates to an optical recording medium for recording and/or reproducing information in which medium an A/W of an optical system is set to 1.0 or lower, where A is an effective radius of an objective lens in the alignment direction of recording tracks and W is a size of a light spot on the objective lens in the alignment direction of the recording tracks. In particular, the present invention relates to an optical recording medium which may be composed to suppress an adverse effect of crosstalk to a minimum by specifying a track pitch.
2. Description of the Related Art
In the field of information recording, recently, optical systems for recording data have been under study in many places. These optical information recording systems have various advantages such as recording or reproducing in a non-contact state, implementation of a higher recording density by one or more digits than a magnetic recording system, and adaptation to various types of memories like a reproduction dedicated type, a direct read after write type, and a rewritable type. In light of these advantages, the optical information recording system makes it possible to implement an inexpensive and a large volume of files and is used in a wide range of applications from industrial uses to domestic uses.
The dedicated reproduction type recording medium contains a digital audio optical disk on which music is recorded, an optical video disk on which picture data is recorded, an optical disk used for a storage unit of a computer and the like. The rewritable type recording medium may be a magneto-optical disk, for example.
These optical disks are more and more frequently required to have higher recording density. As one of the means for achieving the high recording density, the track pitch of the 3recording track is made narrower. In these optical disks, normally, the recording tracks are spirally formed as a recording area. That is, the recording tracks are located in the radial direction of the optical disk so that they are adjacent to each other. Hence, as the radial pitch between the recording tracks, that is, the track pitch is made narrower, more numerous recording tracks may be formed. This makes it possible to enhance the recording density. Typical track pitches adopted as the ISO standards, include for example 1.6 μm (ISO/IEC 10089), and 1.39 μm (ISO/IEC 13549).
In the case of recording and/or reproducing information on and/or from such an optical disk, a recording and/or reproducing ray of light such as a laser beam is applied to a predetermined recording track on the optical disk through an objective lens. As one of the optical system parameters for specifying an intensity distribution of the recording and/or reproducing ray of light on the light-applied plane of the optical disk, it is possible to refer to an A/W (Filling of Lens). If a laser beam (Gaussian beam) is used as the recording and/or reproducing ray of light applied onto the optical disk, the A/W is an index indicating how much the beam is kicked out through the objective lens, in which A denotes an effective radius of the objective lens and W is a spot size of a laser beam on the objective lens.
For example, if the A/W has a value of 1.0, the passage rate of the laser beam through the objective lens is 86.5%. If the MW has a larger value, the spot diameter of the laser beam on the light-applied plane of the optical disk is made narrower. In place, the side lobe of the laser beam is made smaller. If the A/W has a smaller value, the spot diameter is restricted, while the side lobe is made larger.
The spot size of the laser beam (Gaussian beam) on the objective lens depends on an angle of divergence of a laser diode served as a light source. In the initial stage of putting a product into practical use, the angle of divergence is made considerably variable. On the known standards, a considerable allowance is given to the A/W by considering the variety of the manufacturing processes. And, by considering this factor, the known track pitch is defined from a view of a groove-dependent signal, representatively, a push-pull signal or a crosstalk.
However, a recent request has risen for a far higher recording density of the optical disk. Hence, it is necessary to make the spot diameter of the laser beam the light-applied plane of the optical disk narrower. In order to respond to this requirement, the A/W has to be 1.0 or lower. Further, the variety of the A/W is likely to be restricted. Further, in recent days, the accuracy of the laser diode is remarkably improved. The variety of the A/W is more easily restricted. Hence, when defining the track pitch, the necessity for considering the variety of the A/W as a factor is made lower and lower. Further, it is more desirous to make the track pitch narrower and suppress the adverse effect of the crosstalk it a minimum. That is, it is desirous to compose an optical recording medium that makes it possible to enhance the recording density without having any adverse affect on the crosstalk characteristics.